Every summer, clouds gather over India, and how much rain they release determines food for a billion people. But predicting the monsoon's strength in advance was tricky: the best forecasts only gave an answer one month ahead, and often missed the mark. Scientists have found a way to look further ahead, and they were helped by a single point in the ocean, far from India.
The researchers took temperature data from 1948 to 2019 at 726 points around the world. For each pair of points, they calculated how synchronously those points changed, and kept only the strongest links. Then they found the point whose number of incoming links best predicts the rainfall of the next monsoon. That point turned out to be in the South Atlantic, near 30 degrees south latitude.
A test on data from 1989 to 2018 showed that the predicted rainfall matches the actual rainfall with a correlation of 0.5. For comparison, the best models achieved such accuracy only one month before the season, and official forecasts often missed. For 2020, the method predicted 842.85 millimeters of rainfall, which is 5 percent below normal. Now farmers can plan their planting season five months in advance.
The key node in the South Atlantic has warmed by 2.5°C over 70 years, and over the same period its number of incoming links grew from 17 to 32, which improved the forecast.